The development of an expressive language score for people with autism based on the ADOS-2 was recently reported by Mazurek et al. (2019). The current study examined the construct validity of the ADOS-2 expressive language score (ELS) in a sample of adolescents with fragile X syndrome (n = 45, 10 girls), a neurodevelopmental disorder with high rates of autism symptomology. The ADOS-2 ELS showed strong convergent validity with multiple assessments of expressive language, receptive language, and nonverbal cognition. Divergent validity was demonstrated between the expressive language score and chronological age, symptoms of anxiety/depression, and rule-breaking behaviors. This expressive language score is a promising measure of expressive language ability that can be used in research when other language assessments are unavailable.